With a conventional method of fabricating a semiconductor integrated circuit, respective MOSFETs are fabricated by forming isolation regions in a silicon substrate at the start, and subsequently, by forming diffusion regions to serve as a source and a drain, respectively, and a gate electrode covered with a silicon nitride film. Thereafter, a first BPSG film is formed so as to cover those elements, contact holes are defined at predetermined positions in the first BPSG film, respectively, contact plugs are formed by filling up the contact holes with P (phosphorus)-doped polysilicon, respectively, and subsequently, a second BPSG film is deposited on top of the contact plugs, thereby defining holes at predetermined positions where respective capacitors are to be formed. Thereafter, a P-doped polysilicon film to serve as respective lower electrodes is deposited, the P-doped polysilicon film is covered with a protection film such as a resist, and so forth, portions of the P-doped polysilicon film on top of the second BPSG film are removed along with the protection film by etching back, and so forth, and the P-doped polysilicon film is left out only on the inner wall and bottom of the respective holes by removing the protection film inside the respective holes by use of an asher. Portions of the P-doped polysilicon film as left out become the respective lower electrodes.
Next, after cleaning, a thermal nitriding film with an extremely small thickness (on the order of 15 to 20 Å) is formed on the surface of the respective lower electrodes of polysilicon by use of a low pressure CVD (LP-CVD) furnace, and a silicon nitride film is deposited on top of the respective lower electrodes of polysilicon by a CVD method so as to enable a capacitor insulating film to obtain a predetermined thickness. Thereafter, healing oxidation is applied thereto in a wet atmosphere in order to improve the film quality of the capacitor insulating film. Finally, a P-doped polysilicon film to serve as respective upper electrodes is deposited, and predetermined patterning is applied thereto, thereby forming the respective capacitors.
With the conventional method described above, however, at the time of forming the thermal nitriding film on top of the second BPSG film, there occurs incubation on BPSG, and the start of deposition of the silicon nitride film thereon lags behind that on the polysilicon film of the respective lower electrodes, so that the final thickness of the silicon nitride film on top of the second BPSG film becomes smaller than that on the polysilicon film. Consequently, there has arisen a problem in that the lower electrodes and contact plugs have undergone oxidation during healing oxidation in a wet atmosphere, applied later, resulting in failure to form capacitors having desired characteristics.